The invention relates to a solid oxide fuel cell, in particular, a solid oxide fuel cell that is simple in a structure, can generate electricity at a high density, and is easy to handle.
As big problems of the modern world, an energy problem and an environmental problem can be cited. As one of technologies that contribute to overcome the problems, a fuel cell technology is expected and various studies are in progress to achieve higher efficiency and low cost thereof.
The fuel cells are categorized in four types depending on kinds of electrolytes that constitute a battery. That is, 1) a solid polymer type where an electrolyte is a polymer exchange membrane, 2) a phosphoric acid type where an electrolyte is phosphoric acid, 3) a molten carbonate type where an electrolyte is carbonate and 4) a solid oxide type where an electrolyte is a solid oxide can be cited.
The invention relates to the fourth type thereof, that is, a solid oxide fuel cell (hereinafter, in some cases, referred to as “SOFC (solid oxide fuel cell)”).
SOFC
The SOFC uses for instance a ceramic solid oxide electrolyte. Since it can work at such a high temperature as substantially 1000° C., without employing an expensive catalyst such as Pt (platinum), the running cost can be lowered. Furthermore, since the working temperature is high, a temperature of an exhaust heat exhausted when electricity is generated is high as well. Accordingly, when the exhaust heat is used to operate a turbine generator, the power generation due to the SOFC and the turbine power generation can be combined to achieve power generation efficiency of substantially 70%. Since there is no need of externally supplying heat necessary for extracting hydrogen, high efficiency power generation can be realized. Accordingly, as the fuel, other than hydrogen, hydrocarbons in general such as city gas and biomass gas can be advantageously utilized as these are.
Furthermore, since the output density is higher than other batteries, a system can be downsized. In the SOFC, properties in that, when a solid electrolyte formed of ceramics (a stabilized zirconia compound such as YSZ (yttria-stabilized zirconia)) is exposed to a high temperature, an oxide ion can freely pass are utilized to generate electricity.
Cell
A configuration of a cell 2 of the solid oxide fuel cell is shown in FIG. 6. In the solid oxide fuel cell, a cathode electrode layer 23 (expressed by downward-sloping hatchings) is formed on one surface of an oblong planar solid oxide substrate 21, an anode electrode layer 22 (expressed by upward-sloping hatchings) is formed on an opposite surface thereof, and, with the solid oxide substrate 21, the cathode electrode layer 23 and the anode electrode layer 22, one solid oxide fuel battery cell 2 is constituted. On a side of the cathode electrode layer 23, oxygen or oxygen-containing gas is supplied, and, on a side of the anode electrode layer 22, a fuel gas such as methane is supplied. The cell 2 may be formed in circle or other shapes.
Principle of Power Generation
In the next place, the principle of power generation of the solid oxide fuel battery cell 2 will be described.
In the cell 2, oxygen (O2) supplied to the cathode electrode layer 23 is ionized to oxygen ion (O2−) at a boundary surface between the cathode electrode layer 23 and the solid oxide substrate 21 and the oxygen ion moves through the solid oxide substrate 21 to the anode electrode layer 22. At an anode electrode layer 22 interface, the oxygen ion reacts with gas (such as methane (CH4) gas) supplied to the anode electrode layer 22 to generate water (H2O), carbon dioxide (CO2), hydrogen (H2) and carbon monoxide (CO). In the reaction, an electron is released from the oxygen ion.
Here, when external lead wires L1 and L2 are attached to the cathode electrode layer 23 and the anode electrode layer 22, electrons flow from the anode electrode layer 22 through the lead wire to a side of a cathode layer 23 (that is, electricity flows from the cathode electrode layer 23 through the lead wire toward the anode electrode layer 22), thereby power generation can be realized.
Upon generating electricity, the oxygen ion moves inside of the solid oxide substrate 21 to reach the anode electrode layer 22. When a substrate temperature of the solid oxide is low, since the internal resistance becomes larger to be difficult for the oxygen ion to move and for a reaction to occur, the electricity is not generated. In this connection, in order to cause the reaction, the substrate temperature has to be elevated to a power generation temperature in the range of substantially 800 to 1000° C. Accordingly, a perimeter of a cell has to be heated or a fuel has to be burned (patent literature 1).
Constituent Materials of Cell
Subsequently, materials and configurations that constitute the solid oxide substrate 21, the anode electrode layer 22 and the cathode electrode layer 23, which constitute the cell 2, will be described.
In the solid oxide substrate 21, for instance, known materials shown below can be used.                a) YSZ (yttria-stabilized zirconia), ScSZ (scandium-stabilized zirconia) and zirconia-based ceramics obtained by doping Ce or Al thereto,        b) ceria-based ceramics such as SDC (samaria-doped ceria) and GDC (gadolia-doped ceria) and        c) LSGM (lanthanum gallate) and bismuth oxide-based ceramics        
Furthermore, in the anode electrode layer 22, for instance, known materials can be adopted and following materials can be used.                d) Thermet between nickel and yttria-stabilized zirconia, scandia-stabilized zirconia or ceria based (SDC, GDC and YDC) ceramics,        e) sintered bodies with a conductive oxide as a main component (50% by weight or more and 99% by weight or less) (the conductive oxide expresses for instance lithium-dissolved nickel oxide) and        f) ones obtained by compounding 1 to 10% by weight of a metal of platinum group elements or an oxide thereof to ones cited in d) and e) can be cited. Above all, d) and e) are particularly preferred.        
The sintered bodies mainly made of a conductive oxide of e) have excellent oxidation resistance. Accordingly, such phenomena as the deterioration of the power generation efficiency or inability of power generation caused due to a rise in the electrode resistance of the anode electrode layer 22, which is caused due to an oxidation of the anode electrode layer 22 and peeling of the anode electrode layer 22 from the solid oxide substrate 21 can be inhibited from occurring. Furthermore, as the conductive oxide, lithium-dissolved nickel oxide is preferred. Still furthermore, when a metal made of a platinum group element or an oxide thereof is compounded to ones cited in d) or e), high power generation performance can be obtained.
For the cathode electrode layer 23, known materials can be adopted. For instance, manganese oxide compounds (for instance, lanthanum strontium manganite), gallium oxide compounds or cobalt oxide compounds (for instance, lanthanum strontium cobaltite) of the third group element of the periodic table such as lanthanum to which strontium (Sr) is added can be cited.
The cathode electrode layer 23 and the anode electrode layer 22 both are formed in a porous body. In the electrode layers, the open pore rate of the porous body is set at 20% or more, preferably in the range of 30 to 70% and particularly preferably in the range of 40 to 50%. In the solid oxide fuel cell used in the example, it is necessary that the cathode electrode layer 23 and the anode electrode layer 22, which are formed into a porous body, are arranged vertically, and a mixed gas G can go through from a top end thereof toward a bottom end to be supplied over an entire surface of the respective electrode layers.
Producing Method of Battery Cell
In the next place, a producing process of the solid oxide fuel battery cell 2 will be described.
The solid oxide fuel battery cell 2 is produced as shown below.
As the solid oxide substrate 21, a mixture containing samarium-doped ceria (Ce0.8Sm0.2O1.9, hereinafter, referred to as SDC) powder, polyvinyl butyral and dibutyl phthalate is slurried by use of a well known ball mill method, followed by preparing a green sheet having a thickness of substantially 0.2 mm, further followed by punching in a definite shape, still further followed by sintering at 1300° C. in air, and thereby a solid oxide substrate 21 is prepared.
On one surface side of thus obtained solid oxide substrate 21, a paste of a mixture of 50% by weight of samarium/strontium/cobaltite (SSC) and SDC, which becomes the cathode electrode layer 23, is printed. On the other surface thereof, a paste of a mixture of NiO:CoO:SDC at a weight ratio of 50:10:40, which becomes the anode electrode layer 22, is printed. In the sintered body, a platinum mesh (#80) thereto a platinum wire is welded is embedded, followed by sintering at 1200° C. in air to use in the invention, and thereby one sheet of solid oxide fuel battery cell 2 can be produced.
Meshed Metal
Furthermore, as a method of improving the endurance of the solid oxide fuel battery cell 2, a method where a meshed metal is buried in or fastened to the cathode electrode layer 23 and the anode electrode layer 22 is well known.
As a method of burying the meshed metal, there is a method where each of materials (pastes) of the respective layers is coated on the solid oxide substrate 21, followed by burying a meshed metal in the coated material, further followed by sintering. A method of fastening is not to completely bury a meshed metal with a material of each of the layers but to adhere thereto to sinter.
As the meshed metal, ones excellent in harmony with the thermal expansion coefficients of the cathode electrode layer 23 and the anode electrode layer 22 in which or to which the meshed metal is buried or fastened and in the heat resistance are preferred. Specifically, ones obtained by forming a metal made of platinum or an alloy containing platinum into a mesh can be cited.
Furthermore, in place of the meshed metal, a wire-like metal may be buried in or fastened to the cathode electrode layer 23 and the anode electrode layer 22. The wire-like metal is made of a metal same as that of the meshed metal and is not restricted in the number and disposition shape. When the meshed metal or wire-like metal is buried in or fastened to the anode electrode layer 22 and the cathode electrode layer 23, the solid oxide substrate 21 cracked due to the thermal hysteresis can be reinforced so that the solid oxide substrate 21 may not be collapsed in splinters.
The meshed metal or wire-like metal may be buried in either or both of the anode electrode layer 22 and the cathode electrode layer 23. Furthermore, the meshed metal and the wire-like metal may be disposed in combination. In the case of cracks being generated owing to the thermal hysteresis, when the meshed metal or the wire-like metal is buried at least in the anode electrode layer 22, without deteriorating the power generation capability, the power generation can be continued. The power generation capacity of the solid oxide fuel battery cell 2 largely depends on an effective area as a fuel electrode of the anode electrode layer 22; accordingly, the meshed metal or wire-like metal may be disposed at least to the anode electrode layer 22.
Separate Type and Single Chamber Type
Known fuel cells can be divided, from the viewpoint of a gas supply system, into a separate type where an oxygen gas and a fuel gas are supplied through separate paths (patent literatures 2 and 3) and a single chamber type where an oxygen gas and a fuel gas are mixed in advance and supplied (patent literatures 3, 4 and 5 and non-patent literatures 1 and 2). The invention relates to a fuel cell according to the latter type.
In the single chamber type fuel cell, on opposite surfaces of a solid oxide substrate, a cathode electrode layer and an anode electrode layer are disposed to form a fuel battery cell. The fuel battery cell is disposed in a mixed gas G where a fuel gas (such as methane gas) and an oxygen gas are mixed to generate an electromotive force between the cathode electrode layer and the anode electrode layer. Since an entire fuel battery cell can be set in a substantially same atmosphere, a fuel cell can be formed in a single chamber, and thereby the endurance of the fuel battery cell can be improved.    [Patent literature 1] JP-A 2003-297397    [Patent literature 2] JP-A 2005-276519    [Patent literature 3] JP-A 2002-083610    [Patent literature 4] JP-A 2004-199877    [Patent literature 5] JP-A 2003-92124    [Non-patent literature 1] Science, Vol. 288 (2000), p 2031 to 2033    [Non-patent literature 2] Journal of The Electrochemical Society, 149 (2) A133 to A136 (2002)
In existing technologies, even in the single chamber type, a plurality of cells produced according to the foregoing method is directly laminated (or laminated superposed through a separator made of ceramics such as lanthanum chromite or a heat-resistant metal such as SUS based alloy) to form a fuel cell (patent application No. 2005-071645). In a battery having such a configuration, a fuel/air mixed gas G flows through an anode electrode layer and a cathode electrode layer, which are porous bodies; accordingly, the flow resistance becomes large, a width of a speed distribution of the gas flow is large, that is, the gas G does not flow uniformly. Accordingly, the diffusion velocity of the gas G is slow, a chemical reaction is difficult to occur, an internal diffusion overvoltage becomes larger, and thereby the power generation output is small.
Furthermore, reactions in the electrode proceed at interfaces of three phases of the anode electrode layer, the solid oxide substrate and the cathode electrode layer, which are bonded due to the sintering. In the existing technologies, since the anode electrode layer and the cathode electrode layer are laminated, there are irregularities in the flatness of a cell surface and the surface roughness of the electrode. Accordingly, the anode electrode layer and the cathode electrode layer, when coming into contact directly with each other, solely work as an electrical connection point; accordingly, power generation output that can be extracted from the chemical reaction becomes small.
Furthermore, in the existing single chamber type fuel cell, in order to avoid a gas explosion, a gas concentration is managed so as to be in a combustion concentration range. The gas G can be burned within a predetermined concentration range but cannot be burned outside of the range. The predetermined range is called a combustion concentration range. Such a management is very expensive and very dangerous.
As mentioned above, in the existing technology, there are problems in that the power generation output that can be extracted is small and the gas management has to be sufficiently applied.